Tracking bug, this prevents llvm-gcc4 from fully replacing llvm-gcc3.

Jim, 

Could you update this PR with the current status of what remains to be done?

Thanks,

Reid.

Current status:

Thanks to Jim, EH supports is basically runnable. We've successfully run
Shootout-C++/except test on x86/linux. Dunkan is working on some ADA-related
testcases.

There are many small glitches preventing LLVM-side of code to be usable in many
situations, but we're slowly progressing :)

The biggest "bad" thing is lack of support from GCC runtime (this is PR1379),
thus we need to link system (e.g. not compiled by llvm-gcc) libgcc, which
includes working runtime implementation.

As far as I know llvm-gcc-4.2 fully supports C++ exceptions, while
llvm-gcc-4.0 mostly supports them (it doesn't handle some corner
cases correctly).